The invention relates to using scanning interferometry to measure surface topography and/or other characteristics of objects having complex surface structures, such as thin film(s), discrete structures of dissimilar materials, or discrete structures that are underresolved by the optical resolution of an interference microscope. Such measurements are relevant to the characterization of flat panel display components, semiconductor wafer metrology, and in-situ thin film and dissimilar materials analysis.
Interferometric techniques are commonly used to measure the profile of a surface of an object. To do so, an interferometer combines a measurement wavefront reflected from the surface of interest with a reference wavefront reflected from a reference surface to produce an interferogram. Fringes in the interferogram are indicative of spatial variations between the surface of interest and the reference surface.
Typically, a scanning interferometer scans the optical path length difference (OPD) between the reference and measurement legs of the interferometer over a range comparable to, or larger than, the coherence length of the interfering wavefronts, to produce a scanning interferometry signal for each camera pixel used to measure the interferogram. A limited coherence length can be produced, for example, by using a white-light source, which is referred to as scanning white light interferometry (SWLI). A typical scanning white light interferometry (SWLI) signal is a few fringes localized near the zero optical path difference (OPD) position. The signal is typically characterized by a sinusoidal carrier modulation (the “fringes”) with bell-shaped fringe-contrast envelope. The conventional idea underlying SWLI metrology is to make use of the localization of the fringes to measure surface profiles.
SWLI processing techniques include two principle trends. The first approach is to locate the peak or center of the envelope, assuming that this position corresponds to the zero optical path difference (OPD) of a two-beam interferometer for which one beam reflects from the object surface. The second approach is to transform the signal into the frequency domain and calculate the rate of change of phase with wavelength, assuming that an essentially linear slope is directly proportional to object position. See, for example, U.S. Pat. No. 5,398,113 to Peter de Groot. This latter approach is referred to as Frequency Domain Analysis (FDA).
Unfortunately such assumptions may break down when applied to a test object having a thin film because of reflections by the top surface and the underlying film/substrate interface. Recently a method was disclosed in U.S. Pat. No. 6,545,763 to S. W. Kim and G. H. Kim to address such structures. The method fit the frequency domain phase profile of a SWLI signal for the thin film structure to an estimated frequency domain phase profile for various film thicknesses and surface heights. A simultaneous optimization determined the correct film thickness and surface height.
Complex surface structures, e.g. patterned semiconductor wafers, may be comprised of features of dissimilar materials of various sizes from mm down to a few tens of nm in size.
It is presently of considerable interest in the several industries, including in particular the semiconductor industry, to make quantitative measurements of surface topography. Due to the small size of typical chip features, the instruments used to make these measurements typically should have high spatial resolution both parallel and perpendicular to the chip surface. Engineers and scientists use surface topography measuring systems for process control and to detect defects that occur in the course of manufacturing, especially as a result of processes such as etching, polishing, cleaning and patterning.
Non-optical metrology tools such as top down critical dimension (CD) scanning electron microscopy (SEM) and atomic force microscopy (AFM) are in widespread use for obtaining pattern and topography information in the semiconductor industry. While both of these techniques have the required horizontal resolution they suffer from being extremely slow so that it requires significant amounts of time to collect data over large areas of the wafer. This is particularly true of the AFM. Top down CD SEM's are programmable and so can automatically collect data from particular regions of a set of wafers but even with this feature the time required to collect full wafer data is prohibitive.
Conventional optical surface profilers such as confocal, interferometric or slope sensors overcome some of these difficulties; but generally become unusable when surface features are either too small, too closely spaced or both, to be properly resolved and result in inaccurate surface height variations.
Conventional interference microscopes measure surface profiles by directly associating interference phase with an optical path difference between a reference surface and a measurement surface. They have lateral resolution typically limited to approximately one wavelength of the source illumination.
Scanning white light interference microscopes, also known as coherence probe microscopes, laser radar and vertical scanning interferometers, measure surface profiles take advantage of the limited coherence of white light (or more generally, broadband) illumination to assist in surface profiling of discrete surface features, rough surface structures and narrow lines. They have lateral resolution typically limited to approximately one wavelength of the source illumination. Some of these systems may be arranged so as to measure the thickness of films.
Scatterometers determine a surface characteristic by matching the distribution of scattered or diffracted light to a pre-computed library of the scattering and diffraction distributions from nominal structures. They do not directly measure surface profiles with respect to a reference, as in an interferometer. Scatterometers also generally work only with a limited set of 2D structures.
Confocal microscopes use a restricted depth of focus to section an object vertically, so as to e.g. determine surface profile.
Nomarski microscopy and other differential techniques measure differences in surface height by comparing them to each other.
Ellipsometers measure the thin film and dissimilar material structure of objects using polarized light at high angles of incidence and the Fresnel reflection coefficients. Generally the features of interest are large compared to the source wavelength and ellipsometers do not provide surface profile information.